Higher performance, lower cost, increased miniaturization of integrated circuit components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. As these goals are achieved, microelectronic dice become smaller. Accordingly, the density of power consumption of the integrated circuit components in the microelectronic die has increased, which, in turn, increases the average junction temperature of the microelectronic die. If the temperature of the microelectronic die becomes too high, the integrated circuits of the microelectronic die may be damaged or destroyed.
Various apparatus and techniques have been used and are presently being used for removing heat from microelectronic dice. One such heat dissipation technique involves the attachment of a heat dissipation device, such as an integrated heat spreader, high surface area heat sink, heat slug, among others, to the microelectronic die. FIG. 1 is a cross-sectional view of a microelectronic package 1 comprising a microelectronic die 4. (illustrated as a flip chip) physically and electrically attached to a carrier substrate 2, and an integrated heat spreader 6. The integrated heat spreader 6 is thermally coupled to a back surface 5 of the microelectronic die 4 by a thermal interface material 8, such as solder. The integrated heat spreader 6 is usually constructed from a material with a high thermal conductivity, such as copper, copper alloys, aluminum, aluminum alloys, among others. The thermal interface material 8 is by it's composition thermally conductive. The heat generated by the microelectronic die 4 is drawn into the integrated heat spreader 6 through the thermal interface material 8 (following the path of least thermal resistance) by conductive heat transfer.
A critical factor in effectively conducting heat from the microelectronic die 4 to the integrated heat spreader 6 is a good thermally conductive bond or coupling between the back surface 5 of the microelectronic die 4 and the integrated heat spreader 6. The use of thermal interface material 8 between the back surface 5 of the microelectronic die 4 and the integrated heat spreader 6 is a technique to enhance thermal contact between the components. The thermal interface material 8 has the ability to reflow (soften and/or melt) at elevated temperatures and conform to and physically bond with the surfaces in which it is in contact.
One process attempted in the art to reflow the thermal interface material 8 and provide thermal coupling between the microelectronic die 4 and the integrated heat spreader 6 is thermal compression bonding. FIGS. 2A and 2B are partial cross-sectional exploded and assembled views, respectively, of a thermal compression bonding apparatus 10 and microelectronic package 13 known in the art. The microelectronic package 13 comprises a carrier substrate 16 having a substrate active side 19, a microelectronic die 14 (illustrated as a flip chip) having a die active side 21 and a back side 17, a thin sheet or layer of thermal interface material 8, for example a solder preform, and an integrated heat spreader 22. The die active side 21 is physically and electrically coupled to the substrate active side 19 using conventional methods. The reflowable thermal interface material 8 couples the integrated heat spreader 22 to the back side 17 of the microelectronic die 14 by thermal compression bonding.
The thermal compression bonding apparatus 10 consists essentially of a bonding head 11 and a support base 15. In operation, the components of the microelectronic package 13 are placed between the bonding head 11 and a support base 15. The bonding head 11 has a projecting contact surface 12 that provides a continuous surface slightly larger in lateral dimension than the back side 17 of the microelectronic die 14 as well as the thermal interface material 8. The bonding head 11 provides heat to the contact surface 12.
The thermal compression bonding apparatus 10 applies clamping pressure to the microelectronic package 13 between the bonding head 11 and the support base 15. The bonding head 11 is positioned such that the contact surface 12 is in abutment with the heat dissipation device 22 and centered relative to the back side 17 of the microelectronic device 14. Clamping pressure between the bonding head 11 and the support base 15 provides urging engagement between the integrated heat spreader 22, the thermal interface material 8, and the microelectronic die 14. Heat from the bonding head 11 is conducted through the contact surface 12 and through the integrated heat spreader 22 to heat and reflow the thermal interface material 8. The combination of heating and pressure contribute to the formation of the coupling or bond between the integrated heat spreader 22, the thermal interface material 8, and the microelectronic die 14.
In practice, the resulting coupling provided by the thermal interface material 8 between the integrated heat spreader 22 and the microelectronic die 14 is found to be unsatisfactory. Microcracking, embrittlement, and entrapment of solder components, such as flux, in the case of solder, within the thermal interface material 8 is found post thermal compression bonding, all of which are detrimental to thermal conduction. Therefore, thermal compression bonding is not extensively used in the production of microelectronic packages 13.
The benefits of using the thermal compression bonding apparatus 10 include a very short processing time as compared with alternative methods that use clamps to hold the components together while the assembly is heated within a furnace. The thermal compression bonding apparatus 10 can provide the clamping force and heat required for the process. The bonding head 11 can provide the necessary heat much more quickly than the furnace methods. The benefits, though, are outweighed by the inferior coupling produced by the process. Therefore, it is desired in the art to develop thermal compression bonding apparatus and methods to quickly and effectively produce a thermally conductive coupling between the microelectronic die and the integrated heat spreader.